What's the correct molecular formula?

Question

Element analysis reveals that a 270.469g sample, which corresponds to 15.01771 moles, contains 30.2763g of H, What is the correct molecular formula?


i have tried everything from multiplying by avagadros number to dividing the periodic table mass

Answer 1

The molar mass of the compound is 270.469g/15.01771 moles = 18.0100 g/mole

Of course, there is no DEFINITIVE way to answer this without knowing what other elements are present in addition to hydrogen.  But the most obvious solution would be that the correct molecular formula is H₂O, which has a molar mass of 18.0148 g/mol.   

After subtracting the mass of H, you are left with 270.469 g - 30.2763 g = 240.193 g of some other element or combination of elements.  If one assumes it to be oxygen (O), then this corresponds to 240.193 g x 1 mole/15.999g = 15.013 moles O

Moles of H would be 30.2763 g x 1 mole/1.0079 g = 30.039 moles H

Sure enough, they are in a 2:1 ratio, just as in H₂O.